With the advent of communications at optical frequencies, a great effort has been made to provide optical transceiver units that have greater bandwidth capabilities to allow the transmission and reception of audio, digital communication and interactive video signals. Furthermore, because the end user of such modes of communication is in the home or office, there is a requirement that a large number of devices be deliverable that are reliable and yet cost competitive. While the electronics have evolved to achieve the higher data rates and reliability, up until recently the thrust of the product market has focused on bidirectional links that incorporate TO style laser and detector cans. An example of such a device is as disclosed in U.S. Pat. No. 5,127,075 to Althaus, et al, the disclosure of which is specifically incorporated herein by reference. A drawback to a bidirectional link as is disclosed in Althaus, et al. is the required active alignment of the laser/LED and the detector with the focusing and beamsplitting elements in the optical package. This labor intensive alignment results in a bidirectional link that is expensive. Accordingly, passive alignment has become almost required to achieve the cost requirements while maintaining the reliability and data rate requirements.
One technology that has been employed to effect passive alignment between optical components, active devices and fibers is amorphous or monocrystalline silicon selectively etched to provide grooves, pedestals and standoffs to achieve alignment. This technology is known as silicon waferboard technology to the artisan of ordinary skill. There are a variety of alignment techniques using silicon waferboard that have evolved, and are employed in the present invention. To this end, the present invention makes use of etched surfaces in defined orientations, kinematic mounting using microspheres and standoffs or pedestals to effect the passive alignment of the various optical elements.
The first technique employed is the use of defined surfaces and orientations. This allows for the selective etching of grooves and wells for placement of components and reflective surfaces. The etching can be effected on either monocrystalline material where the defined crystalline planes of the material give known orientations for passive alignment or by reactive ion etching of amorphous material to reveal planes in well defined orientations. The former technique is taught for example in U.S. Pat. No. 4,210,923 to North et al., and is specifically incorporated herein by reference. The latter technique is disclosed in U.S. Pat. application Ser. No. 08/251,061 to Boudreau, et al now U.S. Pat. No. 5,519,363.
The second above referenced technique to effect the passive alignment is the use of microspheres mounted in etched wells in the silicon, known as kniematic mounting. This technique allows, among other capabilities, the ability to passively align coplanar substrates, and accordingly is employed in the present invention. Alignment by use of alignment spheres is disclosed in U.S. Pat. application Ser. No. 08/362,625 entitled "Kinematic Mounting of Optical and Optoelectronic Elements on Silicon Waferboard" now U.S. Pat. No. 5,574,561, to Boudreau, et al, the disclosure of which is specifically incorporated herein by reference.
Finally, to properly passively align the active of the submodule, alignment pedestals are employed. These pedestals are generally formed by well known etching techniques, and the techniques and their utility in the alignment of devices is taught in U.S. Pat. No. 5,182,782 to Tabasky et al., the disclosures of which are specifically incorporated herein by reference.
Another technique employed to achieve accurate, passively aligned light focusing is by the use of holographic elements. In particular, holograms are used for directing light in a wavelength selective manner, for example in multiplexing and demultiplexing. Furthermore, the incorporation of holographic optical elements (HOE's) on silicon is a very efficient manufacturing approach. To this end, the fabrication of the holograms on a silicon substrate for wavelength selective direction of the light to and from selected elements of the optical system incorporates the accuracy of the optical elements with the ready fabrication and passively alignment of the elements on a silicon substrate. Holographic elements fabricated on silicon is as disclosed in U.S. Pat. No. 5,420,953 and U.S. Pat. application Ser. No. 08/269,304 to Boudreau, et al. now U.S. Pat. No. 5,500,910 the disclosures of which are specifically incorporated herein by reference.
One particular drawback of bidirectional links up to now, even those using silicon waferboard, has been compactness of the package. For example in the application to Boudreau, et al. above referenced, the separation of wavelengths via the diffractive properties of holograms requires a rather large distance to effect the separated beams. Accordingly, the present invention accomplishes the wavelength isolation in smaller dimensions as compared to conventional wavelength separation techniques by virtue of the use of a dichroic element to achieve the required isolation of the transmit and receive wavelengths. The present invention does achieve the desired precision of beam focus and direction by use of holograms in selected placement.
Accordingly, what is needed is a passively aligned bidirectional link that isolates the transmit and receive wavelengths from one another, and achieves reliable optical signal transmission in compact physical dimensions.